Device with microstructure mediated absorption profile

ABSTRACT

Polymer devices are disclosed with microstructured surfaces that modify their absorption pathway. Polymers which generally degrade in water by fracturing into high surface energy fragments, are modified to degrade in vivo without the formation of sharp fragments. Devices are disclosed that possess improved handling characteristics and degrade in an aqueous environment in a uniform and continuous way that favors the formation of soluble monomers rather than solid particulate. Absorbable medical implants with the disclosed surface modifications are more biocompatible, with reduced foreign body response, and dissolution into metabolizable molecular species.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. provisionalapplication No. 62/574,180, filed on Oct. 18, 2017, and U.S. provisionalapplication No. 62/620,831, filed on Jan. 23, 2018, each of which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates, in general, to surface texturedabsorbable devices, and in particular to devices used in the field ofsurgery and, more particularly, to surgical implants made of absorbablepolymer.

BACKGROUND

While this section is largely devoted to established observations andtheories, some of the material contained in this section is new withrespect to interpretation or perceived application, nevertheless theunderlying theory is known. Thus, the applicants do not intend thatideas disclosed in this section necessarily constitute prior art, andthat some of the connections made between variant states of prior artmay constitute invention, especially with regards to the mechanism ofpromoting surface hydration of microstructured devices.

Fluid pinning is an important way to localize water on specific regionsof a solid substrate. Wetting is the relationship between a liquid phaseand a solid phase, and wetting is essential to fluid pinning. Wetting ischaracterized by a contact angle at the interface between a liquid and asolid surface. The contact angle is representative of the intermolecularinteractions between the liquid and solid wherein the energy ofinteraction is minimized. The contact angle can also be associated witha force balance between adhesive and cohesive forces. Wetting isimportant in the bonding or adherence of two materials.

There are two types of wetting: non-reactive (static) wetting and active(dynamic) wetting. Adhesive force between a liquid and solid cause aliquid drop to spread across the surface of the solid (Wenzel wetting).Cohesive force within the liquid causes the drop to ball up and avoidcontact with the surface (Cassie-Baxter wetting). The juxtaposition ofthe adhesive and cohesive forces results in fluid pinning, which isessentially a balance between the tendency for a fluid to spreadadherently on a surface and the tendency for a fluid to resistattachment and spreading on a surface.

The contact angle is a complicated function of surface texture andchemical composition of the relevant phases. When the contact angle issmall for a liquid on a surface texture, the liquid-solid interface issaid to be in a Wenzel state. When the contact angle is large for aliquid on a surface texture the liquid-solid interface is said to be ina Cassie-Baxter state. When part of a liquid is in a Wenzel state andanother part of the liquid is in a Cassie-Baxter state, the combinedstate is said to be in a Wenzel-Cassie state.

When two solid surfaces form a liquid interface, such as between animplant and a body, surface texture plays a large role in the hydrationof the implant surface. A scale of interaction between a microstructuredsurface and a living surface is defined by the surface texture of themicrostructured device. The microstructure is typically hierarchical,and characterized by at least two spatial scales, one on the order of1-10 micrometers (microns) and another on the order of 10-1000 microns.

It should be appreciated, that in this disclosure, hierarchical meansmicrostructures of different spatial scale. A hierarchicalmicrostructure is defined on a two-dimensional surface characterized bydimensions x and y, and out-of-plane dimension z. Each microstructurescale can be defined by characteristic dimensions x′, y′ and z′ residingon a two-dimensional surface described by function f(x, y). Functionf(x, y) is not necessarily planar. A hierarchical microstructure is aset of scaled microstructures, each characterized by (x′, y′, z′), (x″,y″, z″), and so on; wherein, a first microstructure resides in a regiondefined by (x, y, z1) and a second microstructure resides in a regiondefined by (x, y, z2), and so on. The range z1 spans a range of z valuesdefined by zmin<z1<zmax, and so on. A hierarchical microstructure is athree-dimensional microstructure where most of the first microstructureresides in region z1, and most of the second microstructure resides inregion z2, and so on, such that z1>z2> . . . with respect to anarbitrary set of Euclidean coordinates (x, y, z). For example, ahierarchical microstructure may comprise a set of cylinders of height 10and diameter 2 arranged on the top surfaces of cylinders arranged in aplane of height 100 and diameter 20.

A hierarchical microstructure is self-similar if the ratio of featuredimensions scale by a constant factor. The self-similarity may occur inall of the scale dimensions, or any subset of the scale dimensions. Inthe example of cylinders, the pitch between cylinders at various spatialscales p1, p2, p3, . . . is self-similar, if the pitches satisfy theconstant ratios p1/p2=p2/p3= . . . =c, where c is a constant. Pitch isdefined as the distance between the centers of two like-structures. Inmost cases the pitch is constant for a given type of structure. Aspectratio is a related measure, which is defined as the ratio of the heightof a structure to its width.

Now referring to the composition of implantable materials, bioresorbablepolymers, such as aliphatic polyester polymers derived from lactic acidand glycolic acid are used clinically as sutures, bone fracture fixationdevices and sustained drug delivery systems. The mechanism of theirdegradation in aqueous media is a matter of discussion in theliterature.

PLA/GA polymers are a class of stereo copolymers and copolymers thatdegrade heterogeneously in situ. The degradation in the bulk of thepolymer is faster than at the surface where a layer of less degradedmaterial forms. This has been observed in vitro and in vivo.

In the particular case of intrinsically amorphous PLA/GA materials thephenomenon is particularly significant since polymer degradation leadsto hollow structures when the inner oligomeric residues become solubleand leach out. These hollow structures subsequently fracture, leaving amultiplicity of sharp, high surface energy fragments that then initiatea chronic inflammatory response. This late-time initiation ofinflammation, at a time when the initial insult of the implantation isresiding, is responsible for delayed healing and chronic inflammationleading to necrosis.

Generally speaking, polymeric skin formation after immersion in anaqueous medium is initiated with water uptake. The penetrating waterrapidly creates a negative gradient of water concentration from thesurface to the bulk center line. Degradation then begins from theinside.

Hydrolysis is the most common form of polymer degradation. Solubleoligomers are formed and readily escape from the bulk polymer. Solubleoligomers close to the surface leach out before total degradation,whereas those located well inside the bulk remain entrapped andcontribute significantly to an autocatalytic effect. The resultingdifference in concentration of acidic groups results in the formation ofa skin composed of degradation resistant polymer.

The thickness of the skin depends on many factors such as the diffusionrates of various evolved species and the rate of bond cleavage.Diffusion coefficients of the soluble oligomers depend primarily onmolar mass, degree of polymer swelling in the bulk, and macromolecularconformation and rigidity. Degradation rates can depend on thesequential distribution of chiral and achiral units along the polymerchains. The release of soluble oligomers depends on surface energy,including pH-mediated boundary layers, ionic strength and buffering. Ifthe degrading polymer is or becomes crystalline, bulk degradation canproceed faster that amorphous forms of similar composition.

Paradoxically, the applicants expect implants with thickness less thantwice the critical skin thickness to degrade more slowly and moreuniformly. The unexpected effect is due to the leachable oligomericcompounds escaping before significant auto-catalysis of bulk hydrolysisor enzymatic degradation can take place.

There is anecdotal evidence to support the hypothesis that the greaterthe thickness of an implant the faster the degradation. Grizzi et alfound that large scale devices made of PLA50 degraded faster thansmaller devices, in good agreement with a diffusion-catalysis model.

The terms “degradable”, “bioresorbable” and “absorbable” in the presentcontext are used interchangeably, and apply to polymers thatdisintegrate by a number of processes, including physicaldisintegration, biodegradation by biological mechanisms, and chemicalreaction to monomer units. Materials that undergo only physicaldisintegration have limited utility as implants, since they typicallyelicit prolonged foreign body response. Ideally, the monomers resultingfrom polymer degradation are metabolized by the biological systems intowhich they are implanted.

One approach to preventing polymer skin formation is to modify thesurface properties of the polymer. Surface property modification can beachieved by irradiation, e.g. with laser light, ion and electron beams,UV light, X- and g-rays or treatment in plasma discharge. Irradiationleads to degradation of polymeric chains, chemical bond cleavage,creation of free radicals and release of gaseous degradation products.One disadvantage of surface modification by irradiation is the releaseof transient, highly reactive species. Other undesirable effects areformation of excessive double bonds, production of low mass stabledegradation products, large crosslinked structures and oxidizedstructures.

Another approach to surface modification involves exposure to plasmadischarge. The degree of modification and character of induced changesdepend on the composition of ambient atmosphere, energy of plasma ions,temperature during the treatment and discharge power. Surface propertiesof a polymer can be changed continuously, such as wettability, adhesion,chemical resistance, lubrication or biocompatibility.

Plasma treatment is disadvantageous in several respects. The surfacemodification is typically only tens of micrometers thick. The modifiedsurface properties are quickly reversed in an aqueous environment,making plasma treatment ineffective in implant situations. Plasmatreatment is an extremely complex process requiring precise control ofplasma composition, flow and pressure of the gas, substrate temperature,reactor geometry, discharge power and frequency.

These observations suggest intentional surface texture design maymitigate against polymer skin formation. Surface texture engineering isa known way to permanently modify the surface energy of a polymer.Precise design of the texture on a polymer surface can inhibitfragmented degradation morphologies. In particular, in the manufactureof biocompatible implants, the applicants found matching the surfacetexture to the polymer skin thickness is an important enablingconsideration.

The relationship between polymer surface microstructure and biosorptionkinetics plays a significant role in biocompatibility metrics. Surfacemicrostructure can act as degradation nucleation sites that determinesimplant fractionation and dissolution. Implant degradation that involvessharp, high surface energy particulate formation can trigger a varietyof cellular responses which can adversely affect the biocompatibility ofthe implant. In particular, chronic inflammatory response has beenassociated with implant degradation morphology associated with sharp,solid phase degradation products.

Polymer systems with homogeneous microstructure display simultaneouspolymer degradation and monomer release. The surface texture of apolymer can affect biosorption rates and the shape of the releaseddegradation products. For heterogeneous copolymers, the risk of highsurface energy particulate formation is the greatest. Surface texturingcan confuse the degradation process, and mediate against degradationalong boundaries between heterogeneous regions.

The applicants have discovered that the cellular response mechanism toan implant can be directed by polymer surface microstructure, enhancingthe biocompatibility of the implant.

The degree to which an implant acts as a tissue scaffold and not aforeign body source of irritation plays a pivotal role in an implant'sbiocompatibility. Whenever cell seeding, proliferation, and new tissueformation can be promoted, the degree of reaction oxygen species releaseis diminished. The most biocompatible implants are those that degrade ata rate comparable to the rate of new tissue formation. Surface texturemodification can be used to match degradation rates to cellularinfiltration rates.

New tissue formation is inhibited by the presence of reactive oxygenspecies and cellular infiltrates associated with inflammation. A fewsurface modification techniques such as salt leaching, fibrous fabricprocessing, gas foaming, emulsion freeze-drying, three-dimensionalprinting, and phase separation have been used to enhance cellularinfiltration on implants. However, typically these approaches ignore thedegradation kinetics of the polymeric material comprising the implant.Furthermore, the highly porous nature of many polymer scaffolds fortissue engineering promote particulate formation, that ultimatelyinterferes with healthy tissue recruitment. These scaffolds have showngreat promise in the research of engineering a variety of tissues.However, to engineer clinically useful tissues and organs is still achallenge. The understanding of the principles of polymer surfacetexture and degradation morphology is far from satisfactory.

Surface feature size, connectivity between feature domains, and surfacearea are widely recognized as important parameters for a scaffold fortissue engineering. Other architectural features such as feature shape,hierarchical morphology, and spatial periodicity between features of thescaffolding materials are also believed to be important for cellseeding, migration, growth, mass transport, gene expression, and newtissue formation in three dimensions.

BRIEF SUMMARY

The present invention concerns implants with surface microstructurecapable of 1) minimizing migration of an implant inside a body, 2)promoting healthy tissue infiltration and the avoidance of foreign bodyresponse, 3) exceptional flexibility compared to an implant of the samematerial and thickness without surface microstructure, 4) directing therange of sizes of adverse particulate formation in the case a foreignbody response occurs, and most importantly 5) promoting surfacehydration and inhibiting bulk polymer hydration. The last featurepromotes dissolution of the implant into soluble monomers and inhibitsthe formation of solid phase particulate, in particular the surfacetexture inhibits formation of particulate with high surface energy.

The present invention is an environmentally absorbable device designedto degrade without the formation of particulate. In particular, asurgical implant made of a polymer that degrades while implanted in thetissue of a patient, and the surgical implant has a preferentialhydration zone adapted to degrade at a different rate than the rest ofthe surgical implant. For the embodiments described herein, the surgicalimplant is illustrated as a surgical soft tissue reinforcement devicethat may be formed into a sheet when deployed into tissue, although thepresent invention also is applicable to many other kinds of implants,prostheses, and other polymeric surgical implants. The present inventionis not limited to medical applications, and may be particularly usefulin the manufacture of plastic articles with enhanced environmentaldegradation rates.

The applicants have fabricated implantable polymer surfaces withbioabsorption-directing surface textures to create degradation pathwayswith enhanced biocompatibility. Using well-controlled inter-featureconnectivity, the biodegradable polymer devices of the present inventioncontrol the rate and particulate size of degradation products.

In one embodiment, the preferential hydration zone compriseshierarchical surface microstructures that provides high surface area forsurface-specific hydration.

In another embodiment, the surgical implant has a stacked hierarchicalsurface over at least a portion of its surface, and the preferentialhydration zone comprises a self-similar zone of surface texture.

In another embodiment, the surgical implant develops the preferentialhydration zone during deployment of the surgical implant into the tissueof the patient. In one version of this embodiment, the preferentialhydration zone comprises a microstructure that undergoes hydrolysis orenzymatic degradation preferentially during deployment of the surgicalimplant, thereby causing dissolution of at least a portion of thepolymer of the surgical implant to the tissue of the patient.

In a further embodiment, the surgical implant is a surgical soft tissuereinforcement device that is deployable in physical contact and actingas a surgical barrier against adhesion formation between tissue layersof the patient. In this embodiment the surface microstructure serves tolocalize the implant against migration, and to prevent particulatedegradation of the implant. The applicants have found that ahierarchical surface microstructure can improve the handling propertiesof the implant. For example, surface microstructure reduces thelikelihood of implant folds that often result from trying to fit a flatimplant to a curving tissue surface. The surface microstructuresignificantly enhances the precision of suture placement, and reducesthe force required to pierce the implant.

In one version of this embodiment, the microstructure provides amigration resistant means whereby the surface texture adheres to thepatient tissue.

In another version of this embodiment the surface texture alters thesurface energy of the polymer, and in particular is hydrophilic topromote hydration of a first surface of the device. The first surfacehydrolytically dissolves at an interface between the device and thepatient tissue and does not form solid phase particulate.

An object of the present invention is the manufacture of an absorbablemedical implant having a biocompatible, absorbable core portion and abioabsorbable textured outer surface portion overlying the core portion.

An object of the present invention is a textured implant useful as aprosthesis for tissue augmentation, adhesion barrier, or drug delivery.

An object of the present invention is a biocompatible implant with atextured outer surface portion enveloping the core portion and presentsa high surface area, bioabsorbable textured surface to the exteriorenvironment. As a capsule forms around the implant followingimplantation, the irregular contour of the outer surface of the implantcauses the surface of the implant to hydrate to a greater degree thanthe bulk material thus preventing polymer skin formation which inhibitsfractional disintegration of the implant either during the formation ofthe capsule and/or after the capsule is formed. The outer bioabsorbablesurface portion of the implant is absorbed by the body of the host byreduction of the molecular weight of the polymeric constituents of theimplant, reducing them to soluble, lower molecular weight moieties, andnot to solid phase particulate.

An object of the present invention is to provide a flexible soft tissueaugmentation implant comprised of polylactic acid, polylacticacid/polyglycolic acid copolymer or polyester urethane with surfacemicrostructure capable of 1) minimizing migration of an implant inside abody, 2) promoting healthy tissue infiltration and the avoidance offoreign body response, 3) exceptional flexibility compared to an implantof the same material and thickness without surface microstructure, 4)directing the range of sizes of adverse particulate formation in thecase a foreign body response occurs, and most importantly 5) promotingsurface hydration and inhibiting bulk polymer hydration.

The features of the invention believed to be novel are set forth withparticularity in the appended claims. However the invention itself, bothas to organization and method of operation, together with furtherobjects and advantages thereof, may be best understood by reference tothe following description taken in conjunction with the accompanyingdrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a portion of a first preferredembodiment of an implant in accordance with the present invention.

FIG. 2 is a cross-sectional view of a portion of a second preferredembodiment of an implant in accordance with the present invention.

FIG. 3 is a cross-sectional view of the second preferred embodiment ofan implant in accordance with FIG. 2, and following implantation withinthe body, showing the hydration of the microstructure thereon.

FIG. 4 is a cross-sectional view of the second preferred embodiment ofan implant in accordance with FIG. 3, and following implantation withinthe body, showing the solvation without particulate formation of themicrostructure surface thereon.

FIG. 5 depicts an implantable sheet device comprising a microstructuredsurface as disclosed herein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a medical implant in accordance with thepresent invention is shown in cross-sectional view at numeral 10. Theimplant 10 comprises a (normally extensible) polylactic acid core 11.The elastomeric core 11 has an inner bulk 13 and an outer surface 14.The outer surface 14 of the core 11 is characterized by a plurality ofmicrostructures 15 disposed on the surface portion of the core 11. Themicrostructures 15 are dimensioned to permit the passage of fibroblaststhereinto. The microstructures 15, in accordance with textured outersurfaces 14 are of three types, the first type 17 preferably dimensionedin the range of 1-15 microns in diameter 19 and 10-25 microns in height21. The second type 23 preferably dimensioned in the range of 25-50microns in diameter 25 and 26-50 microns in height 27. The third type 29preferably dimensioned in the range of 100-500 microns in diameter 31and 100-500 microns in height 33. The arrangement of the microstructuresis important because tissue ingrowth requires the migration offibroblast cells into the pores to facilitate deposition of connectivetissue between the microstructures. Such connective tissue, depositedbetween the microstructures, is integral with and part of the structureof the implant. It is believed that the irregular topography of theouter surface 14 of the implant induces non-inflammatory healing in theadjacent tissue layers comprising the surrounding body.

With reference to FIG. 2, a medical implant in accordance with thepresent invention is shown in cross-sectional view of a portion of asecond preferred embodiment of an implant in accordance with the presentinvention at numeral 100. The implant 100 comprises a (normallyextensible) polylactic acid core 110. The elastomeric core 110 has aninner bulk 130 and an outer surface 140. The outer surface 140 of thecore 110 is characterized by a plurality of microstructures 150 disposedon the surface portion of the core 110. The microstructures 150 aredimensioned to permit the passage of endothelial cells thereinto. Themicrostructures 150, in accordance with textured outer surfaces 140 areof three types, the first type 170 preferably dimensioned in the rangeof 1-15 microns in diameter 190 and 10-25 microns in height 210. Thesecond type 230 preferably dimensioned in the range of 25-50 microns indiameter 250 and 26-50 microns in height 270. The third type 290preferably dimensioned in the range of 100-500 microns in diameter 310and 10-50 microns in height 330. Drilled into feature 290 are holes 350of depth equaling height 330 and diameter 10-50 microns 370.

With respect to FIG. 3, a medical implant in accordance with the presentinvention is shown in cross-sectional view of the second preferredembodiment of an implant in accordance with FIG. 2, and followingimplantation within the body, showing the hydration of themicrostructure thereon 200. The implant 210 is immersed in an aqueousenvironment 220. Fine microstructure 240 is hydrophilic and watercompletely fills channels 260. The full volume of microstructure 240 ishydrated 280. Microstructure 300 is hydrophobic and water 310 partiallyfills channels 320. The rest of the channel volume 320 is filled withlipid 330. Only the tips 340 of microstructure 300 are hydrated. Surface350 is resting against tissue, or is coated and hydrates more slowly.Alternatively, the microstructures could be disposed on both sides ofthe implant. Microstructure 360 is made hydrophilic by the inclusion ofdrilled holes or channels 370. The inner surfaces 380 are partiallyhydrated.

With respect to FIG. 4, a medical implant in accordance with the presentinvention is shown in cross-sectional view of the second preferredembodiment of an implant in accordance with FIG. 3, and followingimplantation within the body, showing the solvation 400 withoutparticulate formation of the microstructure surface 402 thereon. Theimplant 402 is entirely encapsulated with healthy, non-fibrogenic tissue404. Remnants of microstructure 406 have the same thickness as the core408 of the implant 402. This state is by design, with the entire implant402 going into dissolution at approximately the same time withoutparticulate formation. The implant 402 is protected from particulateformation by reinforcement from encapsulating tissue 404 and uniformsolvation of the absorbable polymer. Typically, the solvated monomers410 exist as a diffuse soft cloud of monomers, which are metabolized bythe surrounding tissue. Beyond the monomers 410 is water or tissue 412.

FIG. 5 depicts an implantable sheet 500 having a microstructured surface501 on at least a portion thereon. In some embodiments, themicrostructured surface covers the entire surface of the device, whilein others the microstructured surface comprises a portion of the device.Microstructured surface 501 may comprise a preferential hydration zone,as described herein below. The device has a first side 502 and a secondside 503. In some embodiments, the first side comprises themicrostructured surface while the second side is smooth. In otherembodiments, the first and second sides each have a microstructuredsurface, where the surface have either the same or different morphology.The sheet has a core with a thickness 504. In certain embodiments, thesheet comprises a polyester polyurethane, particularly a polyesterpolyurethane as described and exemplified herein.

In the embodiments of a microstructured implant in accordance with thepresent invention, the choice of biodegradable material must be suchthat the structural integrity of the bioabsorbable portion of theimplant is retained for 2-3 months following implantation in order toallow sufficient time for the implant to dissolve from the surface andavoid particulate disintegration.

The implant surface is preferably completely hydrated at the time ofimplantation. This encompasses the inner and outer surfaces of themicrostructures. The hydration can be achieved through textures thatcreate a Wenzel-Cassie interface with tissue. The Wenzel-Cassieinterface binds water to the implant surface. The hydration preferablycomprises at least one water monolayer. A multiplicity of monolayers canalso be layered on top of one another utilizing a stacked hierarchicalsurface texture. Water molecules are bound to the surface texturethrough dipole-dipole bonds and/or through van der Waals forces and/orthrough hydrogen bridges and thus can form extended water moleculelayers.

The use of an implant according to the present invention is foreseen forregulating an adsorption of proteins on the surface of the implant interms of type, quantity and/or conformation of certain proteins by meansof a defined surface, which is at least for the most part hydrated. Thedefined state can also have a defined surface charge and/or a definedpredetermined composition of an oxide layer of the surface. The definedstate is determined according to a desired regulation of the proteinadsorption. Hence for different requirements for protein adsorptiondiffering defined conditions can be established which are each attainedby a suitable surface pattern. Suitable surface patterns develop indifferent location different surface energies, corresponding to separatelocations of hydrophilicity and hydrophobicity. These alternatingpatterns of surface energy can promote certain types of proteinadhesion, and subsequent tissue ingrowth.

Through an implant according to the present invention the quantity ofproteins and other elements adhering on the surface during animplantation of the implant can be changed. For example, undesiredproteins can be reduced and desired proteins settled in an increasedway. More neutrophils can be settled on the implant surface, whichrelease cathelicidin and thus are responsible for a reduction ofrestenosis. The adsorption of thrombocytes can be decreased. Thus therisk of complications with implantation of an implant is significantlyreduced and the growing-together behavior of the implant is improved.Complications from breaking or chipping of coatings on the implant, asis known from the state of the art, are excluded.

Good results have been obtained with implants according to the inventionin which the second surface microstructure is more hydrophobic than thefirst surface microstructure wherein the contact angle of the firstmicrostructure is at least 10% less than the contact angle of the secondmicrostructure, preferably by 20% or more.

Alternatively, the zeta potential value of the surface in the secondmicrostructure should be below the zeta potential value of the firstmicrostructure. The zeta potential can serve to determine a definedstate for the implant surface. The said potential values relate to adetermination procedure by means of electrokinetic analysis. With theuse of other determination procedures, the indications for potentialvalues may possibly have to be adapted according to the proceduralstandards.

In addition to using differences in hierarchical texture to achievesurface energy differences at discrete locations on the implant surfacein order to create water pinning states, e.g., Wenzel-Cassie, suchsurface energy “textures” can promote healthy cellular ingrowth.Ingrowth prevents device mobilization. Implant motion can cause aninflammatory response resulting in the release of reactive oxygenspecies, which then cause rapid degradation of the implant andundesirable implant fragmentation.

Protein deposition on an implant is highly sensitive to surface energy,and can also affect the shape or conformation of adsorbed proteins. Theconformation of proteins adsorbed on an implant surface has an influenceon the adhesion of neutrophils and fibroblasts and thus on thegrowing-together behavior of an implant and the surrounding tissue.Proteins are complex copolymers, whose three-dimensional structure iscomposed of several levels. Involved in the structural composition canbe amino acid sequences, different .alpha.-helix and .beta.-sheetstructures, the common structure of a multiplicity of polypeptides andthe like.

Natural conformation is desired, natural conformation is the shapeproteins take when there are no outside influences affecting thethree-dimensional structure of the proteins and influence the activityof these proteins. To be designated as an almost natural, orrespectively natural-like conformation should be a conformation in whichslight changes in the protein structure exist, but these changes have noinfluence or a negligible influence on the function and effect of theprotein. Proteins comprise different regions, e.g. positively ornegatively charged regions, hydrophilic and hydrophobic regions, which,depending upon spatial organization of the proteins, are exposed and cancarry out specific biological functions. Through adsorption on a surfacethe protein conformation changes. Generally a protein has e.g. on ahydrophobic surface a greatly denatured conformation, while there existson the hydrophilic surface a less denatured conformation. Proteinconformation, because it coats the implant, can affect hydration anddegradation profiles.

The hydrophilic components of the proteins in the natural conformationusually lie outside and the hydrophobic components usually lie insideand are accessible for the hydrophobic surface only through a majorconformational change. Information about the protein conformation can begained through a measurement of the behavior of .alpha.-helix and.beta.-sheets or through a measurement of specific amino acids on theprotein surface.

With the present invention it was surprisingly discovered that e.g.endothelial cells can be settled on an implant surface according to theinvention when fibrinogen is deposited at least approximately in itsnatural, or respectively natural-like, conformation. Endothelial cellinfiltration promotes angiogenesis and healthy tissue association withthe implant.

The efficacy of fibrinogen on an implant surface according to theinvention can be improved, since fibrinogen is adsorbed primarily in anadvantageous conformation. In contrast thereto, fibrinogen on an implantsurface in the unhydrated state is adsorbed in a denatured state,whereby a negative influence on the growing together of an implantresults. In a denatured state fibrinogen has a changed three-dimensionalstructure and a changed spatial distribution of different fibrinogenregions not found in the natural state. A natural conformation also withother proteins promotes a positive growing together of the implant.

The applicants have observed that a natural growing together of tissuepromotes dissolution rather than fragmentation of the implant. Duringthe implantation in a body, the body's own defense or resistance canrecognize the difference between natural and denatured protein, inparticular of fibrinogen, so that denatured protein is identified as aforeign body and an adverse reaction is triggered. Foreign body responseresults in the release of enzymes and reactive oxygen species thatdegrade the implant in a disordered and fragmentary manner.

In use, the amount of adsorbed proteins on the implant can vary in thedefined second state of the surface compared with the starting state ofthe implant surface. For example, the absolute amount of adsorbedproteins can be decreased and/or certain kinds of proteins can beadsorbed in an increased way and other kinds of proteins adsorbed in adecreased way. Thus the risk can be reduced of undesired deposits ofproteins. The type of adhering proteins can thus be regulated in that asuitable defined second state is generated with the hydration and forexample different oxides in the oxide layer or different surface charge.Through the production of an implant with a hydrated surface theadsorption of the proteins can be influenced. Less macroglobulin and/orapolipoprotein A can adhere on the surface and more apolipoprotein E,kininogen and/or plasminogen can be adsorbed. Above and beyond this, theconformation of proteins on the surface can be regulated. For example,fibrinogen can be settled on the implant surface in a way correspondingto its natural conformation, as explained above. Its naturaleffectiveness is thereby preserved and the deposit of endothelial cellspromoted.

Surface Texture Considerations

A critical consideration in achieving the objectives of the presentinvention is to match the volume, areas, depth of penetration ofhydration with the dimensions of the implant. In particular, in regionsof the implant where a large volume to surface area ratio exists,microstructures should be disposed on the boundary surfaces to inhibithydration of the bulk polymer in the region of high volume to surfacearea. This is achieved by a Wenzel-Cassie surface texture which trapswater at a distance separate from surfaces boundary to large volumes.

Other strategies include surface patterns of spatial periodic frequencysuitable for encouraging protein deposition and cellular infiltrationwhich can act as an insulating covering and reinforcement againstimplant fractionation into particulate. If this strategy is to bepursued, it is important that such cellular interaction not cause aninflammatory response, which could result in rapid uncontrolledenzymatic degradation of the implant, especially if such foreign bodyresponse is localized to particular regions of the implant.

Other considerations include the manner in which the microstructuralelements are laid down on the device. In the print construction of animplant, there are two principal modes: a droplet configuration and aline configuration. Drops are discrete in three dimensions, whereaslines are discrete in two dimensions. Lines generally draw water bycapillary action, whereas spherical drops tend to resist water.

In the droplet mode the drops can be spaced apart on a surface, and theycan be joined together by a subsequent layer of drops in staggered form,or joined together by a line. The drops can be spaced closer together toslightly touch, creating an undulating profile, or they can be placed inclose proximity so that they effectively merge before solidifying. Inthe creation of islands, they can be stacked in pyramid fashion in avertical direction.

In the line mode the lines are generally laid down in alignment with theprevious line. However draping configurations can be achieved. Forexample, a partial wall can be formed of several aligned lines on top ofwhich a line is placed such that it crosses this partial wall inundulatory fashion, such that adhesion between the wall and the line isonly at points. After solidification, these draping feature typicallyare free to move away from the established wall structure. Drapingfeatures can be placed at points intermediate during the formation of acomplete wall. In addition, a partial wall can be fenestrated bysubsequent layers of droplets built up to form the edges of windows, thetop edge of which is closed by the subsequent addition of lines. Theselines typically will droop down into the established fenestrations. Byvarying the deposit speed of the final lines one can create amultiplicity of drooping lines into the fenestration of differentlengths creating a curtain of drooping lines.

Alternatively the microstructural elements can be deposited on a planewith a mold pattern. For example, the mold pattern can be asuperhydrophobic pattern capable of generating a Wenzel-Cassie effect ora Wenzel-Baxter effect.

Other surface textures can be achieved by incorporating on or indeposited microstructural element a variety of solid particulate. Thesolid particulate may be a permanent nanostructure, such as ananotubule, a bucky ball, or any of variously known nanoparticulategeometries. The solid particulate may be soluble, such that when thepatterned implant is placed in a solvent the particulate are partiallyor entirely removed without affecting the remaining portion of theimplant.

In addition, directed and random writing techniques can be combined. Forexample, at various points during the construction of a directedstructure a spray or electrospinning technique could be employed todeposit randomly oriented fibrous or particulate masses.

Selection of Polymers

There are many materials that may be used to form a bioabsorbableimplant. For example, the implant may comprise a bioabsorbable materialselected from the group comprising polymers or copolymers of lactide,glycolide, caprolactone, polydioxanone, trimethylene carbonate, polyorthoesters, polyethylene oxide and polyester polyurethane. In additionto the foregoing bioabsorbable, non-toxic materials, high molecularweight polysaccharides from connective tissue such as chondroitin saltsmay be employed for the purpose of practicing the invention. Otherpolysaccharides may also prove suitable, such as chitin and chitosan.Additional bioabsorbable materials are in intense development and it isexpected that many of the new materials will also be applicable forforming a textured bioabsorbable medical implant.

The manufacturing method of the patterned absorbable implant can rely onthe polymer constituents being in a liquid phase. The liquid phase istypically realized by dissolution of a solid polymer in a solvent or bymelting. In the case of a melt phase, it is preferable to selectpolymers having relatively low melting points, to avoid exposingresorbable polymers to elevated temperatures. Resorbable polymers aretypically susceptible to thermal degradation.

A number of polymers are commonly used in the construction ofimplantable medical devices. Unless otherwise specified, the term“polymer” will be used to include any of the materials used to form thepatterned implant matrix, including polymers and monomers which can bepolymerized or adhered at point of application to form an integral unit.

In a preferred embodiment the microstructural elements are formed of apolymer, such as a synthetic thermoplastic polymer, for example,ethylene vinyl acetate, polyanhydrides, polyorthoesters, polymers oflactic acid and glycolic acid and other a hydroxy acids, andpolyphosphazenes, a protein polymer, for example, albumin or collagen,or a polysaccharide containing sugar units such as lactose.

In a more preferred embodiment the polymers are absorbable polyurethanecontaining lactide diol blocks capable of resorbing in vivo. The lactidediol blocks are linked with ethylene diols and/or propylene diols viaurethane or urea links. By varying the proportion of ethylene diol topropylene diol, as well as the choice of the linking diisocyanate, thesurface energy of the resulting polymer can be modified to achieve adesired specification. Generally, these molecules are called polyesterpolyurethanes or polyesters urethanes.

An example of a polyester urethane is an aliphatic polyester based polyester urethane consisting of poly(1-lactic acid) and poly(ethylenesuccinate) prepared via chain-extension reaction of poly(1-lacticacid)-diol and poly(ethylene succinate)-diol using 1,6-hexamethylenediisocyanate as a chain extender. The poly(1-lactic acid)-diol issynthesized by direct polycondensation of 1-lactic acid in the presenceof 1,4-butanediol. Poly(ethylene succinate)-diol can be synthesized bycondensation polymerization of succinic acid with excessive ethyleneglycol.

The polymer is biodegradable via hydrolysis or enzymatic cleavage.Non-polymeric materials can also be used to form the matrix and areincluded within the term “polymer” unless otherwise specified. Examplesinclude organic and inorganic materials such as hydroxyapatite, calciumcarbonate, buffering agents, and lactose, as well as other commonexcipients used in drugs, which are solidified by application ofadhesive rather than solvent. In the case of polymers for use in makingdevices for cell attachment and growth, polymers are selected based onthe ability of the polymer to elicit the appropriate biological responsefrom cells, for example, attachment, migration, proliferation and geneexpression.

An alternative material is a polyester in the polylactide/polyglycolidefamily. These polymers have received a great deal of attention in thedrug delivery and tissue regeneration areas for a number of reasons.They have been in use for over 30 years in surgical sutures, are Foodand Drug Administration (FDA)-approved and have a long and favorableclinical record. A wide range of physical properties and degradationtimes can be achieved by varying the monomer ratios in lactide/glycolidecopolymers: poly-L-lactic acid and poly-glycolic acid exhibit a highdegree of crystallinity and degrade relatively slowly into shards.Copolymers of poly-L-lactic acid and Poly-glycolic acid are amorphousand rapidly degraded into a gel state.

The advantage of the polyester urethane polymers over the polyesterpolymers is that the former degrade both into a gel state and are truesurface-eroding polymer. As a consequence polyester urethanes have apreferred degradation state while retaining for a longer period theoriginal patterns of the implant. However, there are application whereeach are preferred.

The selection of the solvent for chemotaxic agents delivered on aresorbable polymer matrix depends on the desired mode of release of thechemotaxic agent. In the case of a totally resorbable device, a solventis selected to deliver the chemotaxic agent alone and when delivereddissolves the deposited polymer matrix or is selected to contain asecond polymer which is deposited along with the chemotaxic agent.

In the first case, the printed chemotaxic droplet locally dissolves theunderlying polymer matrix and begins to evaporate and thus is adherentto the surface of the immediate underlying polymer matrix layer. In thesecond case, the drug is effectively deposited in the a second polymermatrix after evaporation since the dissolved polymer is deposited alongwith the chemotaxic agent. The first case releases the chemotaxic agentrapidly and creates the highest concentration gradient when placed invivo. The second case releases the chemotaxic agent more slowly sincerelease depends in part of the resorption of the carrier polymer. Inthis second case, the concentration of chemotaxic agent is more uniformand constant over time.

The solvent evaporation rate is primarily determined by the vaporpressure of the solvent. There is a range from one extreme over whichthe polymer is very soluble, for example, 30 weight percent solubility,which allows the polymer to dissolve very quickly, during the timerequired to print one layer, as compared with lower solubilities. Thedegree to which prior layers are dissolved during application of asubsequent layer depends on the solubility of the polymer in thesolvent. Fine fibers are more completely dissolved than fibers withlarger diameters.

Polymer Concentration

In general, microstructural element are a resorbable polymer such aspolyester urethane or polyester of molecular weight 5,000-200,000, in asolvent such as chloroform or a mixture of chloroform and aless-volatile solvent such as ethyl acetate to minimize warping. Thesurface energy of these can be varied by varying the proportion ofhydrophilic and hydrophobic blocks in the polymer. Alternatively, adifferent polymer may be used such as poly-lactic acid, poly-glycolicacid or polycaprolactone.

The polymer concentration in a microstructural element solution willgenerally be at the limit of what can be accommodated by the nozzle,both to maximize the amount of solid polymer delivered and to minimizemigration of the solvent away from the point of application in theformation of a patterned implant. Reduced solvent migration increasesthe resolution of the microstructural elements of prior depositedlayers, e.g., reduces swelling or geometrical slumping.

The upper limit of polymer concentration is 15% for poly-L-lactic acidof 100,000 MW. This concentration of polymer may in some cases makeprinting of commercially viable devices impossible. The cases where thepolymer is sparingly soluble, a filler may be used. Microstructuralelement volume can be increased by including small crosslinked orotherwise less soluble particles in the printing solution.

For example, polyglycolic acid is not soluble in chloroform or ethylacetate. Nanoparticles of crosslinked polyester urethane can be includedin the printing solution (particles up to microns in diameter can beaccommodated through most nozzles) to increase the polymer content whichis printed.

The amount of matter which is printed into the implant can also beincreased by including small inorganic particles in the polymersolution, for example, bone derived apatite.

Manufacturing Methods

A number of processes are known for preparing microstructured molds orextrusion surfaces useful in manufacturing textured polymeric surfaces,e.g., mechanical machining, various lithography techniques, andthree-dimensional printing.

Suitable manufacturing devices include both those with a continuous jetstream print head and a drop-on-demand stream print head. In the formercase, a line of polymer is directed. In the second case, a drop ofpolymer is directed. Pointwise construction of microstructures ispreferred.

A high speed printer of the continuous type, for example, is the Dijitprinter made and sold by Diconix, Inc., of Dayton, Ohio, which has aline printing bar containing approximately 1,500 jets which can deliverup to 60 million droplets per second in a continuous fashion and canprint at speeds up to 900 feet per minute.

Both raster and vector apparatuses can be used. A raster apparatus iswhere the printhead goes back and forth across the bed with the jetturning on and off. This can have problems when the material is likelyto clog the jet upon settling. A vector apparatus is similar to an x-yprinter. Although potentially slower, the vector printer may yield amore uniform finish.

The object of three-dimensional printing is to create a solid stateobject by ink-jet printing a binder into selected areas of sequentiallydeposited layers of powder. In the present disclosure, this process ismodified in that powder is not required. The drop or line that isinitially liquid becomes a volumetric solid when deposited on a surface.In this sense, the process is more like ink in an ink-jet printingprocess, where a third dimension is created by the creation ofsuccessive layer of deposited polymer.

Instructions for each layer can be derived directly from acomputer-aided design (CAD) representation of the patterned implant. Thearea to be printed is obtained by computing the area of intersectionbetween the desired plane and the CAD representation of the object. Afirst layer is joined to a second layer by the liquid state of thepolymer being deposited during the time of creation of the second layer.The liquid state of the second layer partially melts or dissolves intothe first solid layer to form the three dimensional structure insuccessive layers.

While the layers become hardened or at least partially hardened as eachof the layers is laid down, once the desired final implant configurationis achieved and the layering process is complete, in some applicationsit may be desirable that the form and its contents be heated or cured ata suitably selected temperature to further promote binding of thediscrete lines or drops.

Construction of a three-dimensional component by printing can be viewedas the knitting together of structural elements, e.g., drops or lines.These elements are called microstructural primitives. The dimensions ofthe primitives determine the length scale over which the microstructurecan be varied. Thus, the smallest region over which the surface energyof the patterned implant can be varied has dimensions near that ofindividual microstructural primitives. Droplet primitives havedimensions that are very similar to the width of line primitives, thedifference is whether the material is laid down in a continuous line ordiscrete drops. The dimensions of the line primitive depend on thepolymer viscosity and surface tension. A line primitive of 10 micronwidth is in certain cases possible, more typically the dimension is40-60 microns. Higher print head velocities and lower polymer viscosityproduce finer lines.

When solvents are used, the drying rate is an important variable in theproduction of patterned implants by three-dimensional printing. Veryrapid drying of the solvent tends to cause warping of the printedcomponent. Much, if not all, of the warping can be eliminated bychoosing a solvent with a low vapor pressure. For example, patternedimplants prepared by printing with a solution of polymer and chloroformhave nearly undetectable amounts of warpage, while large parts made withmethylene chloride exhibit significant warpage. It has been found thatit is often convenient to combine solvents to achieve minimal warpingand adequate bonding between the particles. Thus, an aggressive solventcan be mixed in small proportions with a solvent with lower vaporpressure.

Bioactive Agents

There are essentially no limitations on the bioactive agents that can beincorporated into the patterned implants of the present invention,although those agents which produce a chemotaxic effect are mostdesirable in wound healing or tissue scaffolding applications. Bioactiveagents need not be incorporated as a liquid, they can be processed intoparticles using spray drying, atomization, grinding, or other standardmethodology, or those agents which can be formed into emulsifications,microparticles, liposomes, or other small particles, and which remainstable chemically and retain biological activity in a polymeric matrix,are useful.

Examples of chemotaxic agents generally include proteins and peptides,nucleic acids, polysaccharides, nucleic acids, lipids, and non-proteinorganic and inorganic compounds. Examples of other bioactive agents havebiological effects including, but not limited to, anti-inflammatories,antimicrobials, anticancer, antivirals, hormones, antioxidants, channelblockers, and vaccines. It is also possible to incorporate materials notexerting a biological effect such as air, radiopaque materials such asbarium, or other imaging agents.

In a preferred embodiment for tissue regeneration matrices, cell growth,differentiation, and/or migration modulators are incorporated intospecific regions of the device at the same level of resolution as thepores and channels. These may act in combination with surface texture,surface energy, and overall shape and distribution of themicrostructural elements to achieve an extracellular matrix mimic withcontrollable tissue directing functionality.

Of particular interest are surface-active agents which promote celladhesion, such as an RGD peptide, or a material which inhibits celladhesion, such as a surfactant, for example, polyethylene glycol or aPluronic (polypropylene oxide-polyethylene oxide block copolymers).

For example, it may be desirable to incorporate adhesion peptides suchas the RGD adhesion peptide into certain channels (e.g., those for bloodvessel ingrowth). An adhesion peptide, such as the peptide having ahydrophobic tail marketed by Telios (La Jolla, Calif.) as Peptide, canbe dissolved in water and deposited onto the surfaces of pores in thepatterned implant.

The surface can be modified to prevent cellular adhesion. This may bedesirable to prevent excessive soft connective tissue ingrowth into thedevice from the surrounding tissue, and can be accomplished, forexample, by depositing an aqueous solution of a pluronic or poloxamer inthe voids. The hydrophobic block of such copolymers will adsorb to thesurface of the channels, with the hydrophilic block extending into theaqueous phase. Surfaces with adsorbed pluronics resist adsorption ofproteins and other biological macromolecules.

In certain embodiments, the patterned implant can contain one or more ofbioactive substance(s) including, but are not limited to, hormones,neurotransmitters, growth factors, hormone, neurotransmitter or growthfactor receptors, interferons, interleukins, chemokines, cytokines,colony stimulating factors, chemotactic factors, extracellular matrixcomponents, and adhesion molecules, ligands and peptides; such as growthhormone, parathyroid hormone (PTH), bone morphogenetic protein (BMP),transforming growth factor-.alpha. (TGF-.alpha.), TGF-.beta.1,TGF-.beta.2, fibroblast growth factor (FGF), granulocyte/macrophagecolony stimulating factor (GMCSF), epidermal growth factor (EGF),platelet derived growth factor (PDGF), insulin-like growth factor (IGF),scatter factor/hepatocyte growth factor (HGF), fibrin, collagen,fibronectin, vitronectin, hyaluronic acid, an RGD-containing peptide orpolypeptide, an angiopoietin and vascular endothelial cell growth factor(VEGF). For example, the patterned implant can include a biologicallyeffective amount of VEGF.

Applications Using Microstructure Implants

In certain embodiments, the patterned implant of the present disclosurecan be implanted in a human subject. For example, in certainembodiments, the patterned implant of the present disclosure can beimplanted in a subject by suturing the patterned implant to fat pads ormuscle tissue in the lower abdomen.

In certain embodiments, the patterned implant of the present disclosurecan be used to enhance vascularization in ischemic settings, such as, byacting as an angiogenic tissue scaffold to promote neovascularizationand ultimately increase blood flow to regions of tissues that are notreceiving sufficient blood supply. In certain embodiments, the patternedimplant of the present disclosure can be implanted in a region of asubject that requires an increase in blood flow. For example, thepatterned implant can be implanted in and/or near an ischemic tissue. Incertain embodiments, the patterned implant can be implanted to treatcardiac ischemia. The patterned implant can be implanted torevascularize from healthy coronary circulation or neighboringnon-coronary vasculature.

In certain embodiments, the patterned implant of the present disclosurecan be used as a novel adjunct to coronary artery bypass grafting (CABG)in addressing cardiac ischemia. In certain embodiments, during CABGsurgery, a surgeon can apply the patterned implant of the presentdisclosure across regions of incomplete reperfusion. For example, thepatterned implant can be placed in order to revascularize from healthycoronary circulation or neighboring non-coronary vasculature (such ascirculation from the left internal mammary artery) into the ischemiczone unlikely to be addressed by the CABG procedure.

In certain embodiments, the patterned implant can be used to directneovascularization around a section of an artery subject to reducedblood flow or occlusion. In this case, the patterned implant can be usedto promote revascularization of a region of ischemic myocardium inaddition to a CABG procedure. In many patients that suffer from acutemyocardial ischemia and in another even larger cohort of patients withuntreatable coronary disease, there remain areas of viable heart that donot naturally revascularized but can be revascularized by an angiogenictissue scaffold. In certain embodiments, the patterned implant canpotentially revascularize those inaccessible ischemic zones in thesepatients. The selection of an alternating hydrophobic/hydrophilicarrangement of fibers of the patterned implant can stimulate andspatially direct revascularization by directing blood flow from nearbyunobstructed coronary vasculature to around and beyond a coronaryobstruction leading to micro-perfused distal myocardium to protectcardiomyocytes viability and function.

The patterned implant of the present disclosure can enhanceneovascularization as well as influence vascular architecture throughtwo potential mechanisms. The patterned implant can be incorporated intoexisting capillary beds to increase blood flow. Second, the patternedimplant can deliver extracellular matrix constituents and secrete growthfactors into tissue thereby providing a microenvironment that promotesangiogenesis.

The patterned implant of the present disclosure is capable of enhancingneovascularization by spatially guiding the invading sprouts of anangiogenic capillary network upon implantation, without incorporationinto the nascent vessels. The patterned implant of the presentdisclosure can be used in conjunction with various types of engineeredtissue constructs to aid in the vascularization of ischemic tissue.

In certain embodiments, the patterned implants of the present disclosurecan be useful in other applications in which it would be beneficial tohave an engineered material to aid in spatially guiding the direction ofhost cell and tissue invasion. Such applications can include, but arenot limited to, nerve regeneration. In certain embodiments, thepatterned implant can be seeded with a heterotypic cell suspension. Forexample, for nerve regeneration applications, the cell suspension caninclude neurons, neuronal stem cells, or cells that are associated withsupporting neuronal function, or a combination thereof. In certainembodiments, the patterned implant can be used at a site of tissuedamage, e.g., neuronal tissue damage.

In certain embodiments, the patterned implant of the present disclosurecan allow for maintenance of the viability and proper function of asurgical repair site. For example, the patterned implant can allow formaintenance of the viability and proper function of muscle tissuesurrounding a hernia repair.

In certain embodiments, the patterned implant of the present disclosurecan enhance wound healing. In certain embodiments, the patternedimplants can be useful in the treatment of chronic wounds such as, forexample, diabetic foot ulcers. Additionally, the patterned implant ofthe present disclosure can be useful in the treatment of woundssustained during military combat. In certain embodiments, the patternedimplant can be implanted in a subject to treat peripheral vasculardisease, diabetic wounds, and clinical ischemia.

In certain embodiments, the patterned implant of the present disclosurecan be used to enhance repair of various tissues. Examples of tissuesthat can be treated by the patterned implant of the present disclosureincludes, but is not limited to, skeletal muscle tissue, skin, fattissue, bone, cardiac tissue, pancreatic tissue, liver tissue, lungtissue, kidney tissue, intestinal tissue, esophageal tissue, stomachtissue, nerve tissue, spinal tissue, and brain tissue.

In certain embodiments, a method of vascularizing a tissue of a subjectincludes providing a patterned implant comprising endothelial cellsorganized along lines and implanting the patterned implant into a tissueof the subject, wherein the implant promotes increased vascularity andperfusion in the subject.

To facilitate a better understanding of the present disclosure, thefollowing examples of certain aspects of some embodiments are given. Inno way should the following examples be read to limit, or define, theentire scope of the disclosure.

Example 1: Polymer

Polymers suitable for constructing patterned implants of the presentdisclosure are preferably absorbable in situ. Polyester Urethanes arepolyurethanes copolymerized with a lactide diol.

Preparation of Lactide Diol

The raw materials:

Compound Source 1,6-Hexanediol Acros Toluene Acros D,L-Lactide SAFCL,L-Lactide Aldrich Tin-ethylhexanoate Sigma Aldrich Chloroform SigmaAldrich Diethylether Sussmann

This procedure is to be performed in closed vessels purged continuouslywith cryogenically distilled (dry) argon or nitrogen. 30 grams of1,6-hexanediol is to be placed in 600 ml of toluene in a graduated 2Liter flat bottom flask equipped with a magnetic stir rod. The flask isto be capped with a 2-hole stopper, one hole equipped with an inputconduit and the other hole equipped with an output conduit connected toan oil trap (to prevent back flow of water vapor). The input conduit isto be connected to the nitrogen source and nitrogen flowed atapproximately 5 Liters per hour. The flask is to be placed on a magneticstirrer/hot top combination.

The toluene solution is to be stirred while raising the solutiontemperature to 70° C., and thereafter in 10° C. increments until thehexanediol is completely dissolved. Upon dissolution, the solutionvolume is to be noted. Temperature and nitrogen flow is to be continueduntil the solution volume drops by 150 ml. Temperature can be raised to130° C. to facilitate toluene vaporization.

A sample of the solution is to be retrieved by syringe (to avoid contactwith humid air), and the toluene removed by vacuum evaporation. A KarlFischer water content measurement is to be performed on the solidhexanediol.

The above distillation procedure is to be continued until the watercontent is <300 ppm H2O by weight. The solution is to be cooled andstored under nitrogen.

Using the above setup, 150 grams of D,L-lactide and 150 grams ofL,L-lactide are to be dissolved in 1750 ml of toluene by heating to 115°C., while stirring under nitrogen flow.

Upon dissolution the solution volume is to be noted and the temperatureis to be raised to 130° C. The nitrogen flow is to be continued until400 ml of toluene is removed.

A sample of the solution is to be retrieved by syringe (to avoid contactwith humid air), and the toluene removed by vacuum evaporation. A KarlFischer water content measurement is to be performed on the solidhexanediol.

The above distillation procedure is to be continued until the watercontent is <300 ppm H2O by weight. The solution is to be cooled andstored under nitrogen.

Weigh an appropriately sized flask (4 L). Note flask weight, preferablythe weight includes closure means or the stopper with closed conduitsdisconnected. The hexanediol and lactide solutions are to be combined inthe weighed flask, connected to nitrogen flow and stirred. The combinedsolution is to be heated in 10° C. increments to 70° C. After 15minutes, 600 mg of tin ethylhexanoate is to be added drop-wise using a 1cc syringe, while stirring vigorously. The temperature of the solutionis to be raised to 120° C. in 10° C. increments. [If a temperaturecontrolled heating mantle is used, the temperature rise will besufficiently slow that the 10° C. heating increment can be ignored.]Turn off the nitrogen flow while keeping conduits connected such thatthe solution volume is closed from contact with air. While stirring andheating, react for 5 hours. Add an additional 400 mg of tinethylhexanoate. Flush with nitrogen. Continue for an additional 3 hours.Add an additional 400 mg of tin ethylhexanoate. Flush with nitrogen.Continue for an additional 11 hours at 120° C. Reduce solutiontemperature to 70° C. Connect the output port of the oil trap to avacuum source. Stop stirring and heat until toluene is removed.Discontinue vacuum. Add 800 ml of dry chloroform flush with nitrogen,stir at 70° C. until the solid is completely dissolved. The resultingturbid solution is to be filtered using a 0.2 micron PTFE filter. Removethe solvent from the filtrate under vacuum.

A sample of the dried solid is to be measured for water content usingKarl-Fischer. The water content is to be <300 ppm. If not within thisspecification, the solid can be dried by chloroform distillation.

Preparation of Polyester Urethane

Raw Materials

Compound Amount of substance IPDI (Isophorone diisocyanate) 202.9 mmol1,4-Butanediol 142.8 mmol Toluene 2000 mL Dibutyltin dilaurate 11.6 mmolPTMG 2000 (Terathane 2000) 20.1 mmol PLA Diol AP1756 40.3 mmol

All operations are to be performed under nitrogen and dry solvents.

Suggested Equipment:

A 2 Liter, four-port graduated glass reactor with central port forintroduction of motor propelled stir rod is recommended. The stir rod ispreferably multi-tier with angled blades to avoid laminar mixing. Thereactor is to be equipped with a heating mantle fitted with athermocouple and a programmable temperature controller. [Preferably, themantle has cooling capability as well, in which a fluid filled mantle isused in conjunction with a circulating control unit.] Preferably thereaction volume is not exposed to the thermocouple, but rather thethermocouple is embedded in the heating mantle. Due to the highviscosity of the final product and need for rapid and complete mixing,use of a magnetic stir rod is discouraged. The two free ports are to beequipped with conduits for delivery and removal of nitrogen. The outputport is to be connected to an oil trap to prevent backflow of watervapor. Ideally the conduits contain valves to provide for transport ofthe reaction volume without exposure to air. The last port, thediagnostic port, is to be used for addition and retrieval of reactionvolume. The nitrogen atmosphere should be delivered at positive partialpressure to compensate for the external stirring means and periodicopening of the diagnostic port. The partial pressure is indicated by theobservation of nitrogen bubbles in the oil trap, and the rate of theircreation can be used to set and maintain a reasonable nitrogen flowrate.

Purge the reactor with nitrogen. Add 40.32 grams of PLA diol, obtainedfrom the procedure above and 40.11 grams of Terathane 2000 and 810 ml oftoluene using the above setup. Set the stir rate to 100 cycles perminute. The dissolution is accomplished by heating to 115° C., whilestirring under nitrogen flow.

Upon dissolution the solution volume is to be noted and the temperatureis to be raised to 130° C. The nitrogen flow is to be continued until200 ml of toluene is removed. Cool the reactor to 15° C. (or roomtemperature, if the mantle is not equipped with coolant). Whilestirring, add via the diagnostic port and under nitrogen flow, 30 mltoluene followed by 45.09 grams of IPDI. Stir for 30 minutes. Add dropwise, 6.74 ml dibutyltin dilaurate.

Using the diagnostic port, remove a sample of the solution to measurethe % NCO. The % NCO can be measured using dibutylamine back titration.By this method, it is traditional to take at least 3 NCO measurements,or you may do so until a desired standard deviation is obtained. Raisethe temperature of the reactor to 75° C. React the mixture undernitrogen flow for 4 hours at 75° C. Take an NCO. React for another 1hour, take an NCO. If the NCO at 5 hours is less than 95% of themeasurement at 4 hours, continue to react for 1 hour durations until theNCO change is less than 5% between consecutive measurements. Using thesetup of the preparation of the PLA diol, dissolve 12.872 g ofbutanediol in 230 ml of dry toluene. Dissolution is accomplished byheating to 75° C. Add the butanediol solution to the reactor. React themixture under nitrogen flow for 9 hours at 75° C. Take an NCO. React foranother 1 hour, take an NCO. If the NCO at 10 hours is less than 95% ofthe measurement at 9 hours, continue to react for 1 hour durations untilthe NCO change is less than 5% between consecutive measurements. Duringthe course of this procedure, toluene may be added to reduce theviscosity of the reactant and improve mixing. Considerable torque candevelop during this reaction. When the NCO has stabilized [this shouldbe reproducible from batch to batch, if not water is entering thesystem], decant the reaction volume to a vacuum chamber. This is easierperformed if the reaction volume is still hot. Apply vacuum and removethe toluene, and the resulting solid is to be dissolved in 1000 ml THF.The polymer is the precipitated in 15 L of pentane, filtered andrepeated washed with pentane and dried under vacuum at 50° C. n-Pentanecan be obtained from Acros and was used after redistillation and THF(also from Acros) was used as received. The resulting polyester urethanehas a melt temperature of 132° C. and is soluble in most solvents, forexample toluene and acetone.

Example 2: Bioactive

All of the synthesis that is detailed below are to be performed in ahermetically sealed glass reactor equipped with a stir rod andtemperature controlled jacket. The headspace of the reactor is to becontinuously flushed with dry nitrogen unless otherwise specified.

Example 2a: Preparation of a Polyester Diisocyanate

In this example a castor-derived hydroxyl-terminated ricinoleatederivative is used as the diol. One equivalent of polycin D-265 (212 g)is combined with 2 equivalent of toluene diisocyanate (174 g) at roomtemperature (22° C.). The mixture is stirred at 100 revolutions perminute and the temperature monitored. The mixture will begin to heat upby exothermic reaction and no heat is to be applied to the reactor untilthe temperature in the reactor ceases to rise. Then the mixturetemperature should be increased in 5° C. increments per ½ hour until themixture reaches 60° C. The reaction should be continued until the %NCO=10.9%. The target % NCO is reached when every hydroxyl group in themixture is reacted with an NCO group. Ideally, the result is a singlediol endcapped with two diisocyanates. This outcome can be enhanced byslow addition of the diol to the diisocyanate. The addition should be in10 g increments, added when the exotherm from the previous addition hasceased. However, chain extended variations of the above ideal outcomeare useful, their primary disadvantage being that the product isslightly higher in viscosity. The ideal % NCO is calculated by dividingthe weight of the functional isocyanate groups (2×42 Dalton) per productmolecule by the total weight of the product molecule (424 Dalton+2×174Dalton) yielding approximately 10.9%.

Alternatively, a lower molecular weight diol may be used, such aspolycin D-290 where 1 equivalent of polycin D-290 is 193 g and thetarget % NCO is 84/(386+348)=11.4%.

Alternatively, a higher molecular weight diol may be used, such aspolycin D-140 where 1 equivalent of polycin D-140 is 400 g and thetarget % NCO is 84/(800+348)=7.3%.

All polycin diols are available from Performance Materials (Greensboro,N.C.) and toluene diisocyanate is available from Sigma-Aldrich(Milwaukee, Wis.).

Example 2b: Preparation of a Polyether Diisocyanate

In this example a polyether hydroxyl-terminated copolymer of 75%ethylene oxide and 35% propylene oxide is used as the diol. Oneequivalent of UCON 75-H-450 (490 g) is combined with 2 equivalent oftoluene diisocyanate (174 g) at room temperature (22° C.). The mixtureis stirred at 100 revolutions per minute and the temperature monitored.The mixture will begin to heat up by exothermic reaction and no heat isto be applied to the reactor until the temperature in the reactor ceasesto rise. Then the mixture temperature should be increased in 5° C.increments per ½ hour until the mixture reaches 60° C. The reactionshould be continued until the % NCO=10.9%. The target % NCO is reachedwhen every hydroxyl group in the mixture is reacted with an NCO group.Ideally, the result is a single diol endcapped with two diisocyanates.This outcome can be enhanced by slow addition of the diol to thediisocyanate. The addition should be in 10 g increments, added when theexotherm from the previous addition has ceased. However, chain extendedvariations of the above ideal outcome are useful, their primarydisadvantage being that the product is slightly higher in viscosity. Theideal % NCO is calculated by dividing the weight of the functionalisocyanate groups (2×42 Dalton) per product molecule by the total weightof the product molecule (980 Dalton+2×174 Dalton) yielding approximately6.3%. Polyether copolymers of ethylene oxide and propylene oxide diolsare available from Dow Chemical (Midland, Mich.).

Example 2c: Preparation of a Polyester Triisocyanate

In this example a castor-derived hydroxyl-terminated ricinoleatederivative is used as the triol. One equivalent of polycin T-400 (141 g)is combined with 2 equivalent of toluene diisocyanate (174 g) at roomtemperature (22° C.). The mixture is stirred at 100 revolutions perminute and the temperature monitored. The mixture will begin to heat upby exothermic reaction and no heat is to be applied to the reactor untilthe temperature in the reactor ceases to rise. Then the mixturetemperature should be increased in 5° C. increments per ½ hour until themixture reaches 60° C. The reaction should be continued until the %NCO=13.3%. The target % NCO is reached when every hydroxyl group in themixture is reacted with an NCO group. Ideally, the result is a singlediol endcapped with two diisocyanates. This outcome can be enhanced byslow addition of the diol to the diisocyanate. The addition should be in10 g increments, added when the exotherm from the previous addition hasceased. However, chain extended variations of the above ideal outcomeare useful, their primary disadvantage being that the product isslightly higher in viscosity. The ideal % NCO is calculated by dividingthe weight of the functional isocyanate groups (2×42 Dalton) per productmolecule by the total weight of the product molecule (282 Dalton+2×174Dalton) yielding approximately 13.3%.

The above reaction will yield a viscous product. A less viscous productcan be obtained by adding propylene carbonate to the initial mixture.Additions up to 100% by weight of propylene carbonate are useful.Adjustment to the target NCO of the mixture must be performed usingstandard methods, or the propylene carbonate may be added after reachingthe target % NCO. Propylene carbonate is available from Sigma-Aldrich(Milwaukee, Wis.).

Example 2d: Preparation of a Polyether Triisocyanate

In this example a polyether hydroxyl-terminated copolymer of 75%ethylene oxide and 35% propylene oxide is used as the triol. Oneequivalent of Multranol 9199 (3066 g) is combined with 3 equivalent oftoluene diisocyanate (261 g) at room temperature (22° C.). The mixtureis stirred at 100 revolutions per minute and the temperature monitored.The mixture will begin to heat up by exothermic reaction and no heat isto be applied to the reactor until the temperature in the reactor ceasesto rise. Then the mixture temperature should be increased in 5° C.increments per ½ hour until the mixture reaches 60° C. The reactionshould be continued until the % NCO=1.3%. The target % NCO is reachedwhen every hydroxyl group in the mixture is reacted with an NCO group.Ideally, the result is a single diol endcapped with two diisocyanates.This outcome can be enhanced by slow addition of the diol to thediisocyanate. The addition should be in 10 g increments, added when theexotherm from the previous addition has ceased. However, chain extendedvariations of the above ideal outcome are useful, their primarydisadvantage being that the product is slightly higher in viscosity. Theideal % NCO is calculated by dividing the weight of the functionalisocyanate groups (3×42 Dalton) per product molecule by the total weightof the product molecule (9199 Dalton+3×174 Dalton) yieldingapproximately 1.3%. Multranol 9199 is available from Bayer (Pittsburgh,Pa.).

Example 2e: Preparation of a Polyol Triisocyanate from Polyol Diol

Any of the diisocyanates prepared in Examples 2a and 2b can betrimerized by the addition of a low molecular weight triol such aspolycin T-400 or trimethylolpropane (TMP). In this example TMP is used,but the method is adaptable to any triol. Complete trimerization of thediisocyanates of Example 2a and 2b will result in viscous products. Toyield a lower viscosity product propylene carbonate can be employed orless triol can be used. In the later case, a mixture of diisocyanate andtriisocyanate is obtained.

In this example the product of Example 2b is used as the polyetherdiisocyanate. One equivalent of Example 2b (682 g) is combined with 0.1equivalent TMP (44.7 g) at room temperature (22° C.). The mixture isstirred at 100 revolutions per minute and the temperature monitored. Themixture will begin to heat up by exothermic reaction and no heat is tobe applied to the reactor until the temperature in the reactor ceases torise. Then the mixture temperature should be increased in 5° C.increments per ½ hour until the mixture reaches 60° C. The reactionshould be continued until the % NCO=5.8%. The target % NCO is reachedwhen every hydroxyl group in the mixture is reacted with an NCO group.The ideal % NCO is calculated by dividing the weight fraction of thefunctional isocyanate groups 10%(3×42 Dalton) and 90%(2×42) per productmolecule by the total weight fraction of the product molecule (3×1364Dalton+134 Dalton)+1364 yielding approximately 0.3%+5.5%=5.8%.

TMP is available from Sigma-Aldrich (Milwaukee, Wis.).

Example 2f: Preparation of a Modified Boswellia Extract Using theTriisocyanate of Example 2d

The hydroxyl number of Boswellia extract will vary depending onextraction method, species of Boswellia extracted, and even variationswithin species. The goal is to obtain a product with no NCOfunctionality, so all reaction mixtures should be reacted until thefinal % NCO=0.

In this example the product of Example 2d is used as the polyethertriisocyanate mixture. One hundred grams of Example 4 is combined with 1g of Boswellia extract at room temperature (22° C.) under 90% nitrogenand 10% nitric oxide atmosphere. The mixture is stirred at 100revolutions per minute and the temperature monitored. The mixture willbegin to heat up by exothermic reaction. When the temperature ceases torise, a % NCO reading is taken. If % NCO>0 then an additional 1 g ofBoswellia extract is to be added. By a series of Boswellia addition onecalculates the change in % NCO as a function of 1 g additions ofBoswellia extract, a linear plot is obtained from which the total amountof Boswellia extract addition necessary to bring the % NCO to zero isobtained. This amount of Boswellia extract is added to the mixture andthe mixture is reacted so that % NCO=0 is obtained.

Example 2g: Preparation of a Modified Boswellia Extract Using theTriisocyanate/Diisocyanate of Example 2e

The hydroxyl number of Boswellia extract will vary depending onextraction method, species of Boswellia extracted, and even variationswithin species. The goal is to obtain a product with no NCOfunctionality, so all reaction mixtures should be reacted until thefinal % NCO=0.

In this example the product of Example 2e is used as the polyetherdiisocyanate/triisocyanate mixture. One hundred grams of Example 2e iscombined with 1 g of Boswellia extract at room temperature (22° C.)under 90% nitrogen and 10% nitric oxide atmosphere. The mixture isstirred at 100 revolutions per minute and the temperature monitored. Themixture will begin to heat up by exothermic reaction. When thetemperature ceases to rise, a % NCO reading is taken. If % NCO>0 then anadditional 1 g of Boswellia extract is to be added. By a series ofBoswellia addition one calculates the change in % NCO as a function of 1g additions of Boswellia extract, a linear plot is obtained from whichthe total amount of Boswellia extract addition necessary to bring the %NCO to zero is obtained. This amount of Boswellia extract is added tothe mixture and the mixture is reacted so that % NCO=0 is obtained.

Example 2h: Preparation of a Highly-Branched Modified Boswellia Extractwith Absorbable Links

Diol and triol can be combined to form a multi-branch polymer. In thisinstance, the Multranol 9199 triol is chain extended with polycin D-265diol. The diisocyanate form of Example 2 is useful in chain extendingthe triisocyanate form of Example 4. We wish to have on average 2diisocyanates for every 3 triisocyanates, which forms a 5 armedisocyanate.

In this example 0.09 equivalents (292 g) of Example 2d is mixed with0.04 equivalents (26.6 g) of Example 2b. The triisocyanates of Example2d and diisocyanates of Example 2b are chain extended with 0.08equivalents lysine diamine to form a 5 armed isocyanate. One hundredgrams of this reaction product is combined with 1 g of Boswellia extractat room temperature (22° C.) under 90% nitrogen and 10% nitric oxideatmosphere. The mixture is stirred at 100 revolutions per minute and thetemperature monitored. The mixture will begin to heat up by exothermicreaction. When the temperature ceases to rise, a % NCO reading is taken.If % NCO>0 then an additional 1 g of Boswellia extract is to be added.By a series of Boswellia addition one calculates the change in % NCO asa function of 1 g additions of Boswellia extract, a linear plot isobtained from which the total amount of Boswellia extract additionnecessary to bring the % NCO to zero is obtained. This amount ofBoswellia extract is added to the mixture and the mixture is reacted sothat % NCO=0 is obtained. Lysine diamine is available from Sigma-Aldrich(Milwaukee, Wis.).

Thus, although there have been described particular embodiments of thepresent invention of a new and useful Device with MicrostructureMediated Absorption Profile it is not intended that such references beconstrued as limitations upon the scope of this invention except as setforth in the following claims.

What is claimed is:
 1. An absorbable polymer medical device comprisingan implant, the implant including a microstructured surface having atleast one morphological feature with a dimension ranging from 5 to 200microns, wherein the implant comprises a polymer that is degradable invivo by hydrolysis or enzymatic degradation, and wherein upondegradation, the implant develops a polymer skin, but said polymer skinis no greater than 100% of the at least one morphological featuredimension.
 2. The device of claim 1, wherein the microstructured surfacecomprises at least two morphological features, wherein the firstmorphological feature comprises i) a height of 5 to 50 microns and ii) awidth of 5 to 50 microns, the second morphological feature comprises i)a height of 50 to 200 microns and ii) a width of 50 to 200 microns, andfurther wherein the first morphological feature is disposed on thesurface of said second morphological feature.
 3. The device of claim 1,wherein the implant is a sheet having a flexural modulus, and theflexural modulus of the implant is less than a flexural modulus of asheet of the polymer that does not comprise the microstructured surface,where the sheets have the same thickness.
 4. The device of claim 3,wherein a force required to puncture the implant with a surgical sutureis less than the force required to puncture the sheet that does notcomprise the microstructured surface.
 5. The device of claim 3, whereinthe implant requires a shear force of translation greater than a sheetthat does not comprise the microstructured surface when implanted in amammalian body.
 6. The device of claim 1, wherein the microstructuredsurface promotes cellular infiltration.
 7. The device of claim 1,wherein the implant degrades into solid fragments having a size smallerthan the largest morphological feature of the microstructured surfaceafter implantation into a mammalian body.
 8. The device of claim 1,wherein the implant does not degrade into solid fragments orparticulates after implantation into a mammalian body.
 9. The device ofclaim 1, the implant further comprising first and second sides, whereinthe first side comprises the microstructured surface and the second sidedoes not.
 10. The device of claim 1, the implant further comprisingfirst and second sides, wherein the first side comprises themicrostructured surface and the second side comprises a secondmicrostructured surface.
 11. The device of claim 1, wherein the implantis adhered to a surgical mesh.
 12. The device of claim 1, wherein thepolymer is selected from the group consisting of polymers of lactide,glycolide, caprolactone, dioxanone, trimethylene carbonate, orthoesters,ethylene oxide, propylene oxide, urethane and combinations thereof. 13.The device of claim 1, wherein the polymer is a polyester polyurethane.14. The device of claim 13, wherein the polyurethane is a copolymercomprising polylactide, a polyester selected from the group consistingof polyethylene oxide; polypropylene oxide; tetrathane; and mixturesthereof.
 15. The device of claim 1, wherein the implant furthercomprises an elastomeric, bioabsorbable core portion having an outersurface comprising the microstructured surface, the microstructuredsurface comprising a plurality of discrete hierarchically arrangedmorphological features arranged periodically such that an exposedportion of the morphological features project outwardly from the outersurface, the exposed portion of said microstructures and said outersurface of said core portion in combination providing an outer surfaceof said implant, wherein the outer surface of said implant has aself-similar topography comprising at least two microstructured scalessuch that upon implantation the exposed portion of the plurality ofmorphological features project outwardly from said outer surface of saidcore portion; and after the morphological features are absorbed, theouter surface of the core portion has a soft partially dissolvedskinless layer thereon.
 16. The device of claim 15, wherein saidelastomeric core portion comprises polylactic acid, polyglycolic acid,or a combination of polylactic acid and polyglycolic acid.
 17. Thedevice of claim 16, wherein said elastomeric core portion comprises apolyester polyurethane.
 18. The device of claim 1, wherein the implanthas at least one preferential hydration zone associated with a firstportion of the surface of said implant adapted to hydrate within theenvironment more quickly than a second portion of the implant, thehydration zone comprising a microtextured surface that modifies thesurface energy of the polymer.
 19. The absorbable device of claim 1,wherein the implant comprises a sheet comprising the polymer, the sheethaving at least one first portion of morphological features disposed ona second portion of morphological features, wherein said first portionhas a spatial periodicity ranging from about 100 to about 500 micronsbetween morphological feature centers, the second portion has a spatialperiodicity ranging from about 100 to 10 microns between morphologicalfeature centers, the sheet ranging in thickness from about 10 microns toabout 2,000 microns, and, upon absorption of a fluid, the secondmorphological features expand geometrically and have a volume expansionranging from about 10% to about 100%, and, in the expanded state, have adensity ranging from about 0.1 g/cm3 to about 1.5 g/cm3.